1. Field of the Invention
The present invention relates generally to light filters, and more particularly to variable color filters used in spotlight projectors.
2. Discussion of the Prior Art
Theatrical productions make wide use of colored spotlight projectors. Such spotlights typically project beams of white light through filter substrates dyed with filtering media of selected color hues and saturations. Two or three different colors of light as components with proportioned brightnesses can be combined to yield intermediate hues and saturations of color.
U.S. Pat. No. 4,535,394 by Dugre can vary the color of light in a beam by adjusting the brightnesses of three individual sources of additive primary color component beams and then joining the component beams through a system of dichroic mirrors. U.S. Pat. No. 3,818,216 by Larraburu can vary the color of light in a beam formed by joining three additive primary color component beams whose intensities are adjusted with continuous gray-scale filters. The component beams are derived by splitting a beam of white light from a single source. U.S. Pat. No. 2,416,919 by Goldsmith for a television camera adjusts three component beam intensities with graded primary additive color filters.
In U.S. Pat. No. 4,294,524 by Stolov, contiguous pixel-like liquid crystal displays in sets of four repeated side-by-side are used as shutters to gate light rays filtered through respective contiguous thin strip filters corresponding to the three additive primary colors and grey in sets of four which filter white light projected from one source. Each set of four strip filters emits rays which, averaged together, can have a selected hue and saturation. The thin strip filter sets being repeated many times across the area of the projected beam lessens the need for different color components to be joined by mirror or lens means, but LCD shutter techniques have the drawback that a significant proportion of the white light penetrates "closed" LCD shutters, de-saturating the hue of the filtered light beam. Stolov also uses a diffuser to commingle the component beams.
U.S. Pat. Nos. 4,459,014 by Thebault, No. 4,600,976 by Callahan, and No. 4,745,531 by Leclercq describe variable color light projecting systems each using three color filter plates having solid areas of respective subtractive primary colors which are insertable one or more at a time to filter proportioned brightnesses of corresponding color components from white light in proportional sub-areas of a beam. However, the filtered areas of each hue and the remaining unfiltered areas of white light are unevenly distributed across the field of the downstream beam, and need to be commingled by an inefficient light-dispersing diffuser.
In U.S. Pat. No. 4,602,321 by Bornhorst, one source projects white light in a beam through dichroic filter elements in three ranks corresponding to respective subtractive primary colors. The filter elements in each rank are adjusted to an angle of incidence to filter their respective primary color in a uniform brightness distribution across the field of the beam. Inconveniently, however, such dichroic filters when adjusted to different angles of incidence do not filter all hues with consistent saturation ratios.
U.S. Pat. No. 3,260,152 by Aston enables color printers to balance print-developing light colors by using three continuous wedge filters of varying densities of respective primary subtractive hue dyes preferably formed on 35 mm film substrates. Aston focuses light through a small area of the film for approximately uniform density filtering. However, photographic film absorbs heat and could not, in small areas, filter the quantities of light required for stage lighting. Furthermore, photographic dye gradients are difficult to reproduce consistently as is required between possibly adjacent stage lights, and the photographic dye grading technique is not readily extendable to gelatin-type substrates. See U.S. Pat. No. 3,943,019 by Krekeler.
Conventional scrolling color changers shift across a light beam an elongated "gelstring" of frames of flexible transparencies ("gels," plastic or polyester) each dyed with a respective preselected hue and saturation of color. The "Panaspot" (tm) changer (by Morpheus Lights, Inc., San Jose, Calif.) uses multiple scrolls of frames, either alternately to facilitate accessing different color frames, or in combination to yield intermediate color hues and/or saturations.
Conventional scrolling color changers, however, can provide only a finite number, on the order of up to several dozen, predetermined color hues and/or saturations. Further, conventional scrolling color changers are obliged either to pass through any intervening color frames in accessing non-adjacent color frames, or else to black-out.
There is, therefore, a need for an improved and more efficient filter means for varying light color over a practically continuous range of selected color hues, saturations and/or brightnesses.